Phosphonitrilate polymers are known to be fire retardants for cellulosic materials: U.S. Pat. No. 3,455,713, Godfrey, Ind. Eng. Chem. Prod. Res. Develop. 9 No. 4 (1970) pgs. 426-436, South African Pat. No. 70/2767, U.S. Pat. No. 2,892,803. Such polymers are made by reacting sodium derivatives of monohydric alcohols with phosphonitrillic chlorides. Alternatively, the monohydric alcohols can be reacted with the phosphonitrillic chloride as described in Netherlands Pat. No. 71/06772.
For some processes for making flame retardant rayon, phosphonitrilates of increased viscosity are desirable. Also, greater retention of the phosphonitrilate polymer in the fiber after forming it and during laundering is desirable. The crosslinked phosphonitrilates of this invention have higher viscosity than phosphonitrilates which are not cross-linked. They also have higher phosphorus content. Other advantageous properties will be apparent from the following description of the invention.